Can a rice cooker be used to boil water?

The electric rice cooker is a common household cooking appliance and is very convenient to use. Because we only need to put rice and water into the electric rice cooker, and the rice will be cooked, and the switch will automatically pop up to disconnect the power or switch to the keep warm state.

01

How does the rice cooker know that the rice is cooked?

In fact, the principle is very simple. In the center of the pot bottom, there is a special material: temperature-sensitive ferrite. Temperature-sensitive ferrite is made by sintering a mixture of manganese oxide, zinc oxide, and iron oxide powder. It is ferromagnetic at room temperature, which means that the magnetism is relatively strong. However, when the temperature rises to about 103 degrees, it will become paramagnetic, and the magnetism is very weak at this time. The temperature of this magnetic transition is called the "Curie point".

When we start cooking, we press the switch. Under the action of the lever, the small magnet on the left end rises and attracts the temperature-sensitive ferrite on the inner pot of the rice cooker. At this time, the elastic copper sheet in the circuit will contact the contact point and connect the circuit. The current passes through the electric heating coil to heat the rice cooker. During the cooking process, since there is water in the pot, the temperature in the pot is always lower than the boiling point of water, 100℃, so the temperature-sensitive ferrite on the inner pot is always magnetic, attracting the permanent magnet to ensure that the circuit is not disconnected.

When the rice is cooked, the water will be absorbed by the rice, so the temperature in the pot will exceed 100 degrees and continue to rise. When the temperature rises to 103 degrees Celsius, the temperature-sensitive ferrite on the inner pot will lose its magnetism. At this time, under the action of the spring, the left end of the lever will fall, driving the insulating sheet to push open the elastic copper sheet and stop heating. Therefore, the rice cooker makes a "bang" sound and the switch pops up, indicating that the rice is cooked. At this time, if we immediately press the switch again, we will find that the switch will pop up again. This is because the temperature in the pot is still very high and the ferrite has no magnetism.

In other words, the temperature-sensitive ferrite can sense the change of temperature and convert the change of temperature into the change of current (on and off) in some way. This device that converts temperature into electrical quantity is called a temperature sensor. The rice cooker uses the temperature sensor to automatically cut off the power after cooking.

So, can you boil water in a rice cooker?

Obviously, using an electric rice cooker can heat water to 100℃ and boil it. However, the temperature of the water will not continue to rise after boiling, so the temperature cannot reach the Curie point of 103℃ and the power cannot be automatically turned off. Either someone needs to watch and manually turn off the power after the water boils, or the electric rice cooker will boil the water dry before it automatically turns off the power. Therefore, it is inconvenient to boil water with an electric rice cooker.

02

Why can the electric kettle cut off the power automatically?

So, what is the principle behind the electric kettle automatically cutting off the power after the water boils?

The temperature sensor it uses is called a "bimetallic temperature sensor".

The principle of this sensor is to stick two metal sheets with different thermal expansion coefficients together, such as iron on one side and copper on the other. When the bimetallic strip is heated, both the iron and copper sheets will expand, but the copper sheet will expand more. As a result, the metal sheet will bend toward the iron sheet.

When the electric kettle boils water, a large amount of water vapor will be generated, and the temperature is around 100 degrees. This water vapor will flow into the bimetallic temperature sensor, causing the bimetallic strip to bend, thereby disconnecting the circuit. However, remember to cover the lid of the kettle when boiling water, because the temperature sensor of many electric kettles is in the lid. If the lid is not covered, the water vapor cannot reach the temperature sensor after the water boils, and the power cannot be automatically disconnected.

03

How to automatically control the temperature of an electric iron?

Another commonly used household appliance: the electric iron also uses this principle to control temperature.

In an electric iron, the heating wire, bimetallic strip, contact and elastic copper sheet form a path. At normal temperature, the two contacts are in contact with each other. When current flows through this path, the heating wire heats up, which can increase the temperature of the iron.

However, when the temperature is too high, due to the different thermal expansion coefficients of the bimetallic strip, the upper metal expands more and the lower metal expands less, so the bimetallic strip bends downward, causing the contacts to separate, thereby cutting off the power supply and stopping heating. After the temperature drops, the bimetallic strip returns to its original state, and the circuit is reconnected for heating. This cycle continues, playing the role of automatic temperature control.

Nowadays, the temperature of many electric irons can be set by yourself, because different temperatures need to be set for ironing clothes made of different materials. There is a temperature adjustment knob in the structure diagram. We can change the set temperature by simply turning this knob.

For example, when ironing cotton and linen clothes, a relatively high temperature needs to be set, which means the contacts need to be in contact more often. Therefore, we can rotate the temperature control knob downward so that the temperature control screw presses deeper against the elastic copper sheet. In this way, the contacts can only separate and the heating can be stopped at a higher temperature.

When ironing silk clothes, a relatively low temperature needs to be set, which requires the contacts to be separated more of the time. Therefore, we can rotate the temperature control knob upwards so that the temperature control screw presses against the elastic copper sheet more shallowly. In this way, when the temperature is not too high, the contacts will separate and the heating will stop.

Temperature sensor is a very common sensor, which makes our life more and more convenient.

END

Source: Mr. Li Yongle